Nominal Capacity vs Actual Capacity

Nominal Capacity

This is the volume of the tank when filled to the brim. Calculated by multiplying the length x width x height e.g. 3m x 2m x 2m = a nominal capacity 12,000 litres.

Actual Capacity

This is the top water level volume when you have taken in to account the amount of water lost when fitting an overflow and warning pipe. If the TWL (top water level) loss was 200mm then you deduct this figure from the overall height. For example: using a 3m x 2m x 2m tank you would calculate the actual capacity of the tank as follows: 3m x 2m x 1.8m = an actual capacity 10,800 litres.

Actual Useable Capacity

This is the available amount of water you can physically use in the tank before you start taking air into the system. It is calculated between two datum points; the top water level volume (see actual capacity) and a dimension at the bottom of the tank, which is either the top of the outlet or the low-level switch (pump protection). This is often referred to as dead water as you technically can’t use it.

Example: How to Calculate Actual Useable Water Tank Capacity

So if this area of water was 300mm you would add this to the TWL figure and then subtract this from the overall height. So using a 3m x 2m x 2m tank you would calculate as follows:

200mm (TWL) + 300mm (Dead water) = 500mm

2000mm (2m) – 500mm = 1500mm

3m x 2m x 1.5m = an actual useable capacity of 9,000 litres

Effective Capacity

This is the available amount of water you can physically use in the tank before you start taking air into the system and is calculated in the same manner as an ‘Actual useable capacity’. However, the term ‘Effective Capacity’ is used by sprinkler companies for sprinkler and wet riser tanks.

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